The success of wireless communications has increased demand for new types of wireless devices as well as for an increase in quantity of these devices. While wireless devices suitable for communication via cell-based systems such as code division multiple access (CDMA) and orthogonal frequency division multiplexed (OFDM) systems were traditionally relegated to telephones, such is no longer the case.
Rather, wireless communication devices also include personal digital assistants (PDAs), pagers, network appliances, laptop and desktop computers, etc. These devices and their users can be divided into three categories, namely, mobile, nomadic and stationary. Mobile devices refer to devices which are moving during use, such as telephones and personal digital assistants when used while walking, riding in a vehicle, and the like. Stationary devices refer to devices which are typically not prone to movement, even during repeated use. An example of a stationary device is a personal tower computer equipped with wireless communication capability. Nomadic devices refer to devices which can move from place to place but are typically stationary during use. Although typically stationary during use, a nomadic device can also be mobile during use. An example of a nomadic device is a laptop computer equipped with wireless communication capability in which the laptop computer is used in an office and moved to another location for subsequent use. According to this example, the laptop computer can be used while being moved, such as while riding in a train or car.
Current wireless communication infrastructures also include one or more base stations, used to communicate with the wireless devices, arranged in a network with access being provided to external services, for example, Internet access. Demand is such that the infrastructure exists in the form of increasing base station and antenna densities, as well as increasing processing loads placed on base station communication equipment as devices are added to the system.
Current and proposed wireless communication environments such as the Third Generation Partnership Project (3GPP) propose different designs for the different categories of devices described above. The device category becomes particularly important when determining the designs for the reverse link (device to base station). In particular, CDMA environments such as the 3GPP propose two types of reverse link designs, namely, synchronous code division multiple access (SCDMA) and asynchronous code division multiple access (ACDMA) with devices operating on one or the other type of link. However, the 3GPP does not address the complementary use of SCDMA codes and ACDMA codes on the reverse link. Further, neither the 3GPP nor the CDMA2000 communication environment address the complementary use of SCDMA codes and ACMDA codes based on the type of device, i.e. stationary or mobile.
SCDMA refers to synchronous orthogonal transmission in which each communication channel is identified by a different orthogonal spreading sequence, and synchronization among channels is achieved by ensuring that transmissions arrive at each receiver at substantially the same time. In contrast, an ACDMA link is a link in which transmissions arrive at the receivers at different times. ACDMA links result in a loss in the orthogonality of the system and an increase in interference within each base station's coverage area, i.e. cell, as compared with a SCDMA link.
As a result of the orthogonality of SCDMA transmission, an orthogonality gain on the order of 3 dB or more with respect to the carrier to interference ratio required to achieve a given bit error rate (BER) over an equivalent ACDMA link is realized. The SCDMA arrangement is desirable over ACDMA operation because the capacity of the carrier channel is therefore increased when all devices are operating synchronously. However, the presence of devices operating out of time alignment, i.e. asynchronously with the other devices, increases interference in the channel, thereby decreasing capacity and performance in the channel. As discussed above, SCDMA links require time alignment among the receivers and also require the use of orthogonal-spreading codes such as Walsh-spreading codes. Because the number of codes in an orthogonal-spreading code environment are limited as compared with non-orthogonal codes such as those used in ACDMA links, the number of devices which can simultaneously be used with a particular carrier within a cell are limited. This limitation makes the code assignment aspect particularly important and therefore increases system complexity.
An integral feature of CDMA systems is the concept of soft handover. Soft handover refers to the simultaneous communication between a wireless device and multiple base stations such that communication is transferred from one base station to another in a make-before-break fashion, i.e., communication is established with the new base station prior to breaking the communication link with the current base station. A device using an SCDMA
code can maintain soft handoff with other base stations, however, these other base stations see the SCDMA code as an ordinary pseudo-noise code. Therefore, devices in soft handoff increase the amount of interference experienced by the SCDMA devices within the cell. Because accurate time alignment, e.g. within one-eighth or one-quarter of a chip, is required in a high-capacity SCDMA system, devices in the mobile category described above have difficulty maintaining synchronous operation on the reverse link due to device movement with respect to other devices and the base station. Furthermore, the ability to maintain synchronization is also impacted by the variability in fading and interference, even for stationary uses. This adverse effect is particularly prevalent in wide-band (i.e. 5 MHz and up) systems due to the very fast chip rates involved. As a result, systems such as those conforming to 3GPP standards propose separate designs for low and high mobility devices.
The category in which a device operates may change, for example, when a mobile device user stops moving for a prolonged period. Current systems do not, however, support the switch between one category and another, i.e., ACDMA to/from SCDMA reverse link operation. As such, a mobile device which becomes stationary may be relegated to less efficient and lower capacity asynchronous operation when, in fact, the device could make efficient use of an SCDMA reverse link. Similarly, a nomadic device which changes from stationary to mobile operation may adversely impact the performance of an originally assigned SCDMA reverse link due to its imposition of increased channel interference due to the inability to maintain orthogonality.
It is therefore desirable to have a wireless communication system which allows a device to operate in an SCDMA reverse link mode or an ACDMA reverse link mode depending on the particular profile of the wireless device during operation.
Further, because current wireless communication systems do not facilitate switching from ACDMA to SCDMA operation and vice-versa during a communication session, these current systems do not monitor the operational mode of the device to detect operational category changes of the device, e.g. the ability of the device to maintain synchronization (degree of unit mobility). Also, current systems are not optimized for situations which restrict the use of an SCDMA code.
It is therefore desirable to have a system which monitors the operational characteristics, i.e. category of operation, of the devices to detect a change therein. This is particularly the case in multi-carrier wide-band operation in which a system provider allocates their wide-band frequency spectrum into multiple discrete carriers such that each carrier supports a particular type of operation, for example SCDMA or ACDMA.
It is also desirable to have a CDMA system which reduces interference to devices operating in an SCDMA mode while offering the use of unlimited ACDMA codes for mobile users to ensure optimal system operation.
Use of a plurality of discrete frequency division multiplexed (FDM) carriers as described in U.S. patent application Ser. No. 09/797,273 allows separate carriers to support ADCMA and SCDMA codes, but is not as efficient as using a single large bandwidth carrier to support both code types due to the bandwidth wasted as a result of the need to provide a guard band between each separate carrier. It is therefore further desirable to have a method of supporting SCDMA codes and ADCMA codes in a communication environment which uses a single large bandwidth carrier as opposed to a plurality of smaller carriers.
The additional capacity gains in SCDMA relative to ADCMA access are obtained at the expense of code limitations. As such, the number of wireless devices which can access a base station at any one time are severely limited in the case of SCDMA access. It is therefore desirable to have a system and method which provides a way to reuse spreading codes within a cell (as used herein, “cell” refers to the communication area supported by a base station).
Although theoretically optimal, not all wireless devices communicating with a base station will be orthogonal with respect to one another. This is due to a number of factors. First, channel conditions and/or velocity of the wireless device may inhibit accurate time alignment at the base station. Second, some users in a CDMA system will be in soft handoff. As a result, the wireless device signal arrival time can be time aligned with no more than one base station. Third, because there are a limited number of orthogonal spreading codes available at each base station, wireless devices communicating with the same base station may begin to reuse these codes after scrambling them differently. As such, the codes will appear as pseudonoise codes to other wireless devices. The result is that the transmission of some of the wireless devices will be orthogonal to each other and some will not. Of course, the greater the number of wireless devices transmitting orthogonally with each other, the greater the capacity of the channel.
It is desirable therefore, to have a system and method which can optimize wireless communication channels by grouping the transmission from wireless devices which are transmitting orthogonally to each other.
Many wireless systems include radio resource managers (“RRMs”), also known as schedulers. Among other functions, RRMs operate to manage the wireless communication channels for a base station or group of base stations by assigning time slots, frequencies and spreading codes to the wireless devices associated with the base station(s). These assignments are typically based on channel conditions such as channel quality (C/I ratio), but can also be assigned based on quality of service requirements, wireless device communication priority and/or a round robin assignment scheme.
As used herein, a time slot represents the unit of time which serves to divide the sharing of transmission resources in the time domain. Typically, such time slots are quite short, for example, on the order of one millisecond. A device may be granted transmission resources for one or more consecutive time slots. After this time period has passed, another device may be granted the same transmission resources. However, the transmissions from the two devices are separated in time because they are transmitting in separate time slots. As such the devices do not interfere with each other.
However, current RRMs do not include support for tracking whether time slots are allocated to spreading codes for ACDMA or SCDMA communication. Known RRMs also cannot group wireless devices into those using/requiring ADCMA communication, i.e. wireless devices not in orthogonal communication with other wireless devices and those wireless devices using/requiring SCDMA communication, i.e. wireless devices in orthogonal communication with other wireless devices, in order to facilitate time slot assignment which maximizes channel capacity. The result is that time slots associated with a particular channel are assigned in a non-optimal fashion, thereby leading to inefficient use of the channel and a reduction in channel and system capacity.
It is therefore also desirable to have a CDMA system in which RRMs can track whether time slots correspond to SCDMA codes or ACDMA codes and group wireless devices in a manner such that devices operating in a mode which require an ACDMA code are grouped together for purposes of time slot assignment and devices operating in a mode which can use an SCDMA code are grouped together for purposes of time slot assignment, thereby maximizing the efficiency of the channel and system. It is further desirable to have a method and system which allows the limited spreading codes corresponding to SCDMA communication with a base station within a cell to be reused in a manner which preserves communication orthogonality between large groups of wireless devices.